Methods Apparatus for Manufacturing Geometric Multi-Crystalline Cast Materials

ABSTRACT

Methods are provided for casting one or more of a semi-conductor, an oxide, and an intermetallic material. With such methods, a cast body of a geometrically ordered multi-crystalline form of the one or more of a semiconductor, an oxide, and an intermetallic material may be formed that is free or substantially free of radially-distributed impurities and defects and having at least two dimensions that are each at least about 10 cm.

This application claims the benefit of priority from U.S. ProvisionalApplication No. 06/951,927, filed Jul. 25, 2007, the entirety of whichis expressly incorporated herein by reference.

This invention was made with U.S. Government support under NationalRenewable Energy Laboratory (NREL) Subcontract No. ZDO-2-30628-03 underDepartment of Energy (DOE) Contract No. DE-AC36-98GO10337, awarded byDOE. The U.S. Government has certain rights in this invention.

TECHNICAL FIELD

The present invention generally relates to the field of photovoltaicsand to methods and apparatuses for manufacturing cast silicon forphotovoltaic applications. The invention further relates to new forms ofcast silicon that can be used to manufacture devices, such asphotovoltaic cells and other semiconductor devices. The new silicon canhave a geometrically ordered multi-crystalline structure and can bemanufactured by a casting process.

BACKGROUND INFORMATION

Photovoltaic cells convert light into electric current. One of the mostimportant measures of a photovoltaic cell is its efficiency inconverting light energy into electrical energy. Although photovoltaiccells can be fabricated from a variety of semiconductor materials,silicon is generally used because it is readily available at reasonablecost, and because it has a suitable balance of electrical, physical, andchemical properties for use in fabricating photovoltaic cells.

In a known procedure for the manufacture of photovoltaic cells, siliconfeedstock is mixed with a material (or dopant) for inducing either apositive or negative conductivity type, melted, and then crystallized byeither pulling crystallized silicon out of a melt zone into ingots ofmonocrystalline silicon (via the Czochralski (CZ) or float zone (FZ)methods), or cast into blocks or “bricks” of multi-crystalline siliconor polycrystalline silicon, depending on the grain size of theindividual silicon grains. In the procedure described above, the ingotsor blocks are cut into thin substrates, also referred to as wafers, byknown slicing or sawing methods. These wafers may then be processed intophotovoltaic cells.

Monocrystalline silicon for use in the manufacture of photovoltaic cellsis generally produced by the CZ or FZ methods, both being processes inwhich a cylindrically shaped boule of crystalline silicon is produced.For a CZ process, the boule is slowly pulled out of a pool of moltensilicon. For a FZ process, solid material is fed through a melting zoneand re-solidified on the other side of the melting zone. A boule ofmonocrystalline silicon, manufactured in these ways, contains a radialdistribution of impurities and defects such as rings of oxygen-inducedstacking faults (OSF) and “swirl” defects of interstitial or vacancyclusters. Even with the presence of these impurities and defects,monocrystalline silicon is generally a preferred source of silicon forproducing photovoltaic cells, because it can be used to produce highefficiency solar cells. Monocrystalline silicon is, however, moreexpensive to produce than conventional multi-crystalline silicon, usingknown techniques such as those described above.

Conventional multi-crystalline silicon for use in the manufacture ofphotovoltaic cells is generally produced by a casting process. Castingprocesses for preparing conventional multi-crystalline silicon are knownin the art of photovoltaic technology. Briefly, in such processes,molten silicon is contained in a crucible, such as a quartz crucible,and is cooled in a controlled manner to permit the crystallization ofthe silicon contained therein. The block of multi-crystalline siliconthat results is generally cut into bricks having a cross-section that isthe same as or close to the size of the wafer to be used formanufacturing a photovoltaic cell, and the bricks are sawed or otherwisecut into such wafers. The multi-crystalline silicon produced in suchmanner is an agglomeration of crystal grains where, within the wafersmade therefrom, the orientation of the grains relative to one another iseffectively random.

The random orientation of grains, in either conventionalmulti-crystalline or poly-crystalline silicon, makes it difficult totexture the surface of a resulting wafer. Texturing is used to improveefficiency of a photovoltaic cell, by reducing light reflection andimproving light energy absorption through the surface a cell.Additionally, “kinks” that form in the boundaries between the grains ofconventional multi-crystalline silicon tend to nucleate structuraldefects in the form of clusters or lines of dislocations. Thesedislocations, and the impurities they tend to attract, are believed tocause a fast recombination of electrical charge carriers in afunctioning photovoltaic cell made from conventional multi-crystallinesilicon. This can cause a decrease in the efficiency of the cell.Photovoltaic cells made from such multi-crystalline silicon generallyhave lower efficiency compared to equivalent photovoltaic cells madefrom monocrystalline silicon, even considering the radial distributionof defects present in monocrystalline silicon produced by knowntechniques. However, because of the relative simplicity and lower costsfor manufacturing conventional multi-crystalline silicon, as well aseffective defect passivation in cell processing, multi-crystallinesilicon is a more widely used form of silicon for manufacturingphotovoltaic cells.

Some previous casting techniques involved using a “cold-wall” cruciblefor crystal growth. The term “cold-wall” refers to the fact thatinduction coils present on or in the walls of the crucible are watercooled, and may also be slotted, thus generally remaining below 100° C.The crucible walls may be situated in close proximity between the coilsand the feedstock. The material of the crucible walls is notparticularly thermally insulating, and can therefore remain in thermalequilibrium with the cooled coils. The heating of the silicon istherefore not predicated on radiation from the crucible walls, becauseinductive heating of the silicon in the crucible means that the siliconis heated directly by current induced to flow therein. In this way, thewalls of the crucible remain below the melting temperature of thesilicon, and are considered “cold,” relative to the molten silicon.During solidification of the inductively heated molten silicon, thesecold walls of the crucible act as a heat sink. The ingot cools quickly,determined by radiation to the cold walls. Therefore, an initialsolidification front quickly becomes substantially curved, with crystalnucleation occurring at the ingot sides and growing diagonally towardsthe ingot center, disrupting any attempt at maintaining a vertical andgeometrically ordered seeding process or a substantially flatsolidification front.

In view of the foregoing, there is a need for an improved form ofsilicon that can be used to manufacture photovoltaic cells. There isalso a need for silicon that can be manufactured in a process that isfaster and less expensive than the processes that have been heretoforeused to produce monocrystalline silicon. The present invention providessuch silicon and such processes.

SUMMARY OF THE INVENTION

As used herein, the term “monocrystalline silicon” refers to a body ofsingle crystal silicon, having one consistent crystal orientationthroughout. Further, conventional multi-crystalline silicon refers tocrystalline silicon having cm-scale grain size distribution, withmultiple randomly oriented crystals located within a body of silicon.

Further, as used herein, the term “poly-crystalline silicon” refers tocrystalline silicon with micron order grain size and multiple grainorientations located within a given body of silicon. For example, thegrains are typically an average of about submicron to submillimeter insize (e.g., individual grains may not be visible to the naked eye), andgrain orientation distributed randomly throughout.

Still further, as used herein, the term “near-monocrystalline silicon”refers to a body of crystalline silicon, having one consistent crystalorientation throughout for greater than 50% by volume of the body,where, for example, such near-monocrystalline silicon may comprise abody of single crystal silicon next to a multicrystalline region, or itmay comprise a large, contiguously consistent crystal of silicon thatpartially or wholly contains smaller crystals of silicon of othercrystal orientations, where the smaller crystals do not make up morethan 50% of the overall volume. Preferably, the near-monocrystallinesilicon may contain smaller crystals which do not make up more than 25%of the overall volume. More preferably, the near-monocrystalline siliconmay contain smaller crystals which do not make up more than 10% of theoverall volume. Still more preferably, the near-monocrystalline siliconmay contain smaller crystals which do not make up more than 5% of theoverall volume.

As used herein, however, the term “geometrically orderedmulti-crystalline silicon” (hereinafter abbreviated as “geometricmulti-crystalline silicon”) refers to crystalline silicon, according toembodiments of the present invention, having a geometrically orderedcm-scale grain size distribution, with multiple ordered crystals locatedwithin a body of silicon. For example, in geometric multi-crystallinesilicon, each grain typically has an average cross-sectional area ofabout 0.25 cm² to about 2,500 cm² in size, where the cross-section is inthe plane perpendicular to the height or length of the grain, and aheight that can be as large as the body of silicon, for example, theheight can be as large as the dimension of the body of silicon that isperpendicular to the plane of the cross-section, with grain orientationwithin a body of geometric multi-crystalline silicon being controlledaccording to predetermined orientations. The shape of the cross-sectionof the grain that is perpendicular to the height or length of the grainof geometric multi-crystalline silicon is typically the same as theshape of the seed crystal or part of a seed crystal over which it wasformed. Preferably, the shape of the cross-section of the grain ispolygonal. Preferably, the corners of the polygonal grains correspond tojunctions of three different grains. Although each grain within a bodyof geometric multi-crystalline silicon preferably comprises siliconhaving one contiguously consistent crystal orientation throughout thatgrain, one or more grains can also contain small amounts of smallercrystals of silicon of different orientation. For example, each suchgrain can partially or wholly contain smaller crystals of silicon ofother crystal orientations, where such smaller crystals do not make upmore than 25% of the overall volume of the grain, preferably not morethan 10% of the overall volume of the grain, more preferably not morethan 5% of the overall volume of the grain, still more preferably notmore than 1% of the overall volume of the grain, and still morepreferably not more than 0.1% of the overall volume of the grain.

In accordance with the invention as embodied and broadly described,there is provided a method of manufacturing cast silicon, comprising:placing a geometric arrangement of a plurality of monocrystallinesilicon seed crystals on at least one surface in a crucible having oneor more side walls heated to at least the melting temperature of siliconand at least one wall for cooling; placing molten silicon in contactwith the geometric arrangement of monocrystalline silicon seed crystals;and forming a solid body of geometrically ordered multi-crystallinesilicon, optionally having at least two dimensions each being at leastabout 10 cm, by cooling the molten silicon to control crystallization,wherein the forming includes controlling a solid-liquid interface at anedge of the molten silicon during the cooling so as to move in adirection that increases a distance between the molten silicon and theat least one wall for cooling. It is contemplated that one of the wallsof the crucible may be a bottom of the crucible.

In accordance with an embodiment of the present invention, there is alsoprovided a method of manufacturing cast silicon, comprising: arranging aplurality of monocrystalline silicon seed crystals in a predeterminedpattern on at least two surfaces of a crucible having one or more sidewalls heated to at least the melting temperature of silicon and at leastone wall for cooling; placing molten silicon in contact with theplurality of monocrystalline silicon seed crystals; and forming a solidbody of geometrically ordered multi-crystalline silicon, optionallyhaving at least two dimensions each being at least about 10 cm, bycooling the molten silicon from the at least two surfaces of thecrucible to control crystallization, wherein the forming includescontrolling a solid-liquid interface at an edge of the molten siliconduring the cooling so as to move the interface in a direction thatincreases a distance between the molten silicon and the monocrystallinesilicon seed crystals in the crucible.

In accordance with another embodiment of the present invention, there isalso provided a method of manufacturing cast silicon, comprising:placing a geometric arrangement of a plurality of monocrystallinesilicon seed crystals on at least one surface in a crucible; placingsilicon feedstock in contact with the plurality of monocrystallinesilicon seed crystals on the at least one surface; heating the siliconfeedstock and the plurality of monocrystalline silicon seed crystals tothe melting temperature of silicon; controlling the heating so that theplurality of monocrystalline silicon seed crystals does not meltcompletely, the controlling comprising maintaining a ΔT of about 0.1°C./min or less, as measured on an outside surface of the crucible, afterreaching the melting temperature of silicon elsewhere in the crucible;and, once the plurality of seed crystals are partially melted, forming asolid body of geometrically ordered multi-crystalline silicon by coolingthe silicon.

In accordance with a further embodiment of the present invention, thereis also provided a method of manufacturing cast silicon, comprising:arranging a plurality of monocrystalline silicon seed crystals in apredetermined pattern on at least two surfaces of a crucible; placingsilicon feedstock in contact with the plurality of monocrystallinesilicon seed crystals on the at least two surfaces; heating the siliconfeedstock and the plurality of monocrystalline silicon seed crystals tothe melting temperature of silicon; controlling the heating so that theplurality of monocrystalline silicon seed crystals does not meltcompletely, the controlling comprising maintaining a ΔT of about 0.1°C./min or less, as measured on an outside surface of the crucible, afterreaching the melting temperature of silicon elsewhere in the crucible;and, once the plurality of seed crystals are partially melted, forming asolid body of geometrically ordered multi-crystalline silicon by coolingthe silicon.

In accordance with an embodiment of the present invention, there is alsoprovided a method of manufacturing cast silicon, comprising: placing atleast one geometric multi-crystalline silicon seed crystal on at leastone surface in a crucible having one or more side walls heated to atleast the melting temperature of silicon and at least one wall forcooling; placing molten silicon in contact with the at least one seedcrystal; and forming a solid body of geometrically orderedmulti-crystalline silicon, optionally having at least two dimensionseach being at least about 10 cm, by cooling the molten silicon tocontrol crystallization, wherein the forming includes controlling asolid-liquid interface at an edge of the molten silicon during thecooling so as to move in a direction that increases a distance betweenthe molten silicon and the at least one geometric multi-crystallinesilicon seed crystal in the crucible.

In accordance with another embodiment of the present invention, there isalso provided a method of manufacturing cast silicon, comprising:placing a geometric arrangement of a plurality of monocrystallinesilicon seed crystals on at least one surface in a crucible, theplurality of monocrystalline silicon seed crystals arranged to cover anentire or substantially an entire area of the at least one surface inthe crucible; placing molten silicon in contact with the geometricarrangement of monocrystalline silicon seed crystals; and forming asolid body of geometrically ordered multi-crystalline silicon,optionally having at least two dimensions each being at least about 10cm, by cooling the molten silicon to control crystallization.

In accordance with another embodiment of the present invention, there isalso provided a method of manufacturing cast silicon, comprising:placing molten silicon in contact with at least one geometricmulti-crystalline silicon seed crystal in a vessel having one or moreside walls heated to at least the melting temperature of silicon, the atleast one geometrically ordered multi-crystalline silicon seed crystalarranged to cover an entire or substantially an entire area of a surfaceof the vessel; and forming a solid body of geometrically orderedmulti-crystalline silicon, optionally having at least two dimensionseach being at least about 10 cm, by cooling the molten silicon tocontrol crystallization.

In accordance with a further embodiment of the present invention, thereis also provided a body of continuous geometrically orderedmulti-crystalline silicon having a predetermined arrangement of grainorientations, the body optionally further having at least two dimensionsthat are each at least about 10 cm and a third dimension at least about5 cm.

In accordance with yet another embodiment of the present invention,there is also provided a body of continuous cast geometrically orderedmulti-crystalline silicon having a predetermined arrangement of grainorientations, the body optionally having at least two dimensions thatare each at least about 10 cm.

In accordance with a still further embodiment of the present invention,there is also provided a continuous geometrically orderedmulti-crystalline silicon wafer having a predetermined arrangement ofgrain orientations, the wafer further having at least two dimensionsthat are each at least about 50 mm.

In accordance with a still further embodiment of the present invention,there is also provided a solar cell, comprising: a wafer formed from abody of continuous geometrically ordered multi-crystalline silicon, thebody having a predetermined arrangement of grain orientations preferablywith a common pole direction being perpendicular to a surface of thebody, the body further having at least two dimensions that are eachoptionally at least about 10 cm and a third dimension at least about 5cm; a p-n junction in the wafer; an optional anti-reflective coating ona surface of the wafer; optionally at least one layer selected from aback surface field and a passivating layer; and electrically conductivecontacts on the wafer.

In accordance with a still further embodiment of the present invention,there is also provided a solar cell, comprising: a wafer formed from abody of continuous cast geometrically ordered multi-crystalline silicon,the body having a predetermined arrangement of grain orientationspreferably with a common pole direction being perpendicular to a surfaceof the body, the body further having at least two dimensions that areeach optionally at least about 10 cm; a p-n junction in the wafer; anoptional anti-reflective coating on a surface of the wafer; optionallyat least one layer selected from a back surface field and a passivatinglayer; and electrically conductive contacts on the wafer.

In accordance with a still further embodiment of the present invention,there is also provided a solar cell, comprising: a continuousgeometrically ordered multi-crystalline silicon wafer having apredetermined arrangement of grain orientations preferably with a commonpole direction being perpendicular to a surface of the wafer, the waferfurther having at least two dimensions that are each at least about 50mm; a p-n junction in the wafer; an optional anti-reflective coating ona surface of the wafer; optionally at least one layer selected from aback surface field and a passivating layer; and electrically conductivecontacts on the wafer.

In accordance with a still further embodiment of the present invention,there is also provided a wafer, comprising: silicon formed from a bodyof continuous geometrically ordered multi-crystalline silicon, the bodyhaving a predetermined arrangement of grain orientations preferably witha common pole direction being perpendicular to a surface of the body,the body further having at least two dimensions that are each optionallyat least about 10 cm and a third dimension at least about 5 cm.

In accordance with a still further embodiment of the present invention,there is also provided a wafer, comprising: silicon formed from a bodyof continuous cast geometrically ordered multi-crystalline silicon, thebody having a predetermined arrangement of grain orientations preferablywith a common pole direction being perpendicular to a surface of thebody, the body further having at least two dimensions that are eachoptionally at least about 10 cm.

In accordance with a still further embodiment of the present invention,there is also provided a wafer, comprising: a continuous geometricallyordered multi-crystalline silicon wafer having a predeterminedarrangement of grain orientations preferably with a common poledirection being perpendicular to a surface of the wafer, the waferfurther having at least two dimensions that are each at least about 50mm.

In accordance with a still further embodiment of the present invention,there is also provided a solar cell, comprising: a wafer sliced from abody of continuous geometrically ordered multi-crystalline silicon, thebody having a predetermined arrangement of grain orientations preferablywith a common pole direction being perpendicular to a surface of thebody, the body further having at least two dimensions that are eachoptionally at least about 10 cm and a third dimension at least about 5cm; a p-n junction in the wafer; an optional anti-reflective coating ona surface of the wafer; at least one optional layer selected from a backsurface field and a passivating layer; and a plurality of electricallyconductive contacts on at least one surface of the wafer.

In accordance with a still further embodiment of the present invention,there is also provided a solar cell, comprising: a wafer sliced from abody of continuous cast geometrically ordered multi-crystalline silicon,the body having a predetermined arrangement of grain orientationspreferably with a common pole direction being perpendicular to a surfaceof the body; the body further having at least two dimensions that areeach optionally at least about 10 cm; a p-n junction in the wafer; anoptional anti-reflective coating on a surface of the wafer; at least oneoptional layer selected from a back surface field and a passivatinglayer; and a plurality of electrically conductive contacts on at leastone surface of the wafer.

In accordance with a still further embodiment of the present invention,there is also provided a solar cell, comprising: a continuousgeometrically ordered multi-crystalline silicon wafer having apredetermined arrangement of grain orientations preferably with a commonpole direction being perpendicular to a surface of the wafer, the waferfurther having at least two dimensions that are each at least about 50mm; a p-n junction in the wafer; an optional anti-reflective coating ona surface of the wafer; at least one optional layer selected from a backsurface field and a passivating layer; and a plurality of electricallyconductive contacts on at least one surface of the wafer.

In accordance with another embodiment of the present invention,near-monocrystalline silicon made according to the invention can containup to 5% by volume of smaller crystals of silicon of other crystalorientations. Preferably, in accordance with another embodiment of thepresent invention, near-monocrystalline silicon made according to theinvention can contain up to 1% by volume of smaller crystals of siliconof other crystal orientations. Still more preferably, in accordance withanother embodiment of the present invention, near-monocrystallinesilicon made according to the invention can contain up to 0.1% by volumeof smaller crystals of silicon of other crystal orientations.

Additional features and advantages of the invention will be set forth inthe description that follows, being apparent from the description orlearned by practice of embodiments of the invention. The features andother advantages of the invention will be realized and attained by thesemiconductor device structures and methods and apparatuses ofmanufacture particularly pointed out in the written description andclaims, as well as the appended drawings.

It is to be understood that both the foregoing general description andthe following detailed description are exemplary and explanatory, andare intended to provide further explanation of the invention as claimed.This invention also includes silicon made by the methods described andclaimed herein, and wafers and solar cells made from such silicon.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are incorporated in and constitute apart of this specification, illustrate embodiments of the invention and,together with the description, serve to explain the features,advantages, and principles of the invention. In the drawings:

FIG. 1 illustrates an exemplary arrangement of silicon seeds on thebottom surface of a crucible, according to an embodiment of the presentinvention;

FIG. 2 illustrates another exemplary arrangement of silicon seeds on thebottom and side surfaces of a crucible, according to an embodiment ofthe present invention;

FIG. 3A-3C illustrate an example of tiling for casting geometricallyordered multi-crystalline silicon in a crucible, according to anembodiment of the present invention;

FIG. 4 illustrates another example of tiling for casting geometricallyordered multi-crystalline silicon in a crucible, according to anembodiment of the present invention;

FIG. 5 illustrates an example of a close-packed array of hexagon seedtiles, according to an embodiment of the present invention;

FIG. 6 illustrates an exemplary array of polygonal shapes havingrhomboid or triangular interstices, according to an embodiment of thepresent invention

FIG. 7 illustrates an exemplary method, according to an embodiment ofthe present invention; and

FIGS. 8A-8G and 9 illustrate exemplary casting processes formonocrystalline or geometrically ordered multi-crystalline silicon,according to embodiments of the present invention.

DESCRIPTION OF THE EMBODIMENTS

Reference will now be made in detail to embodiments of the presentinvention, examples of which are illustrated in the accompanyingdrawings. Wherever possible, the same or similar reference numbers willbe used throughout the drawings to refer to the same or like parts.

In embodiments consistent with the invention, the crystallization ofmolten silicon is conducted by casting processes using one or more seedcrystals. As disclosed herein, such casting processes may be implementedso that the size, shape, and orientation of crystal grains in the castbody of crystallized silicon is controlled. As used herein, the term“cast” means that the silicon is formed by cooling molten silicon in amold or vessel used to hold the molten silicon. Since a liquid, such asmolten silicon, will take the shape of the container in which it isplaced, it is also contemplated herein that the cooling of moltensilicon may also be accomplished while confining the molten silicon byany means, and not just in a mold or vessel. By way of example, thesilicon can be formed by solidification in a crucible, wheresolidification is initiated from at least one wall of the crucible, andnot through a cooled foreign object introduced into the melt. Thecrucible may have any suitable shape, such as a cup, a cylinder, or abox. Thus, the process of molten silicon crystallization according tothis invention is not controlled by “pulling” a boule or ribbon.Further, consistent with an embodiment of the present invention, themold, vessel, or crucible includes at least one “hot side wall” surfacein contact with the molten silicon. As used herein, the term “hot sidewall” refers to a surface that is isothermal with or hotter than themolten silicon that it contacts. Preferably, a hot side wall surfaceremains fixed during processing of the silicon.

Consistent with embodiments of the invention, the crystallized siliconcan be either continuous monocrystalline, near-monocrystalline silicon,or continuous geometric multi-crystalline having controlled grainorientations. As used herein, the term “continuous monocrystallinesilicon” refers to single crystal silicon, where the body of silicon isone homogeneous body of monocrystalline silicon and not smaller piecesof silicon joined together to form a larger piece of silicon. Further,as used herein, the term “continuous geometric multi-crystallinesilicon” refers to geometric multi-crystalline silicon where the body ofsilicon is one homogeneous body of geometric multi-crystalline siliconand not smaller pieces of silicon joined together to form a larger pieceof silicon.

Consistent with embodiments of the present invention, thecrystallization can be accomplished by positioning a desired collectionof crystalline silicon “seeds” in, for example, the bottom of a vessel,such as a quartz crucible that can hold molten silicon. As used herein,the term “seed” refers to a preferably geometrically shaped piece ofsilicon with a desired crystal structure, preferably wherein at leastone cross-section has a geometric, preferably polygonal, shape, andpreferably having a side that conforms to a surface of a vessel in whichit may be placed. Such a seed can be either a monocrystalline piece ofsilicon or a piece of geometrically ordered multi-crystalline silicon,for example, a slab or horizontal section cut or otherwise obtained froman ingot of geometrically ordered multi-crystalline silicon. Consistentwith the present invention, a seed may have a top surface that isparallel to its bottom surface, although this does not have to be thecase. For example, a seed can be a piece of silicon, varying in sizefrom about 2 mm to about 3000 mm across. For example, a seed can beabout 10 mm to about 300 mm across. The piece of silicon may have athickness of about 1 mm to about 1000 mm, preferably about 5 mm to about50 mm. A suitable size and shape of the seed may be selected forconvenience and tiling. Tiling, which will be described in more detailbelow, is where silicon seed crystals are arranged in a predeterminedgeometric orientation or pattern across, for example, the bottom or oneor more of the sides and the bottom surfaces of a crucible. It ispreferable that the seed or seeds cover the entire crucible surface nextto which they are located, so that when moving the seeded crystal growthsolidification front away from the seeds, the full size of the cruciblecross-section can be maintained as a consistent geometric crystal.

The molten silicon is then allowed to cool and crystallize in thepresence of the seeds, preferably in a manner such that the cooling ofthe molten silicon is conducted so that the crystallization of themolten silicon starts at or below the level of the original top of thesolid seeds and proceeds away, preferably upwards away, from the seeds.The solid-liquid interface at an edge of the molten silicon willpreferably initially conform to a cooling surface of the vessel, such asa surface in a crucible, in which it is being cast. According toembodiments of the invention, the liquid-solid interface between themolten silicon and the crystallized silicon can be maintainedsubstantially flat throughout part, for example, the initial part of thesolidification stage, or all of the casting process. In an embodiment ofthe invention, the solid-liquid interface at each of the edges of themolten silicon is controlled during the cooling so as to move in adirection that increases a distance between the molten silicon and thecooled surface of the crucible while preferably maintaining asubstantially flat solid-liquid interface.

Therefore, consistent with the present invention, the solidificationfront may parallel the shape of a cooled surface of the vessel. Forexample, with a flat-bottomed crucible, the solidification front mayremain substantially flat, with the solid-liquid interface having acontrolled profile. The solid-liquid interface can be controlled so thatits radius of curvature decreases as one moves from the edge to thecenter. Alternatively, the solid-liquid interface can be controlled tomaintain an average radius of curvature of at least half the width ofthe vessel. Moreover, the solid-liquid interface can be controlled tomaintain an average radius of curvature of at least twice the width ofthe vessel. The solid can have a slightly convex interface with a radiusof curvature at least about four times the width of the vessel. Forexample, the solid-liquid interface can have a radius of curvaturegenerally greater than 2 m in a 0.7 m square crucible, more than twicethe horizontal dimension of the crucible, and preferably about 8× toabout 16× a horizontal dimension of the crucible.

According to embodiments of the present invention, a solid body ofmonocrystalline silicon, or near-monocrystalline silicon, preferablycast, preferably having at least two dimensions each being at leastabout 20 cm, for example, at least about 20 cm on a side, and a thirddimension at least about 10 cm, can be formed. Preferably, a solid bodyof monocrystalline silicon, or near-monocrystalline silicon, preferablycast, having at least two dimensions each being at least about 30 cm,for example, at least about 30 cm on a side, and a third dimension atleast about 10 cm, can be formed. More preferably, a solid body ofmonocrystalline silicon, or near-monocrystalline silicon, preferablycast, having at least two dimensions each being at least about 35 cm,for example, at least about 35 cm on a side, and a third dimension atleast about 10 cm, can be formed. Still more preferably, a solid body ofmonocrystalline silicon, or near-monocrystalline silicon, preferablycast, having at least two dimensions each being at least about 40 cm,for example, at least about 40 cm on a side, and a third dimension atleast about 20 cm, can be formed. Still more preferably, a solid body ofmonocrystalline silicon, or near-monocrystalline silicon, preferablycast, having at least two dimensions each being at least about 50 cm,for example, at least about 50 cm on a side, and a third dimension atleast about 20 cm, can be formed. Still more preferably, a solid body ofmonocrystalline silicon, or near-monocrystalline silicon, preferablycast, having at least two dimensions each being at least about 60 cm,for example, at least about 50 cm on a side, and a third dimension atleast about 20 cm, can be formed. Still more preferably, a solid body ofmonocrystalline silicon, or near-monocrystalline silicon, preferablycast, having at least two dimensions each being at least about 70 cm,for example, at least about 70 cm on a side, and a third dimension atleast about 20 cm, can be formed.

An upper limit of horizontal size of an ingot of cast silicon madeaccording to embodiments of the invention is only determined by castingand crucible making technology, and not by the invented method itself.Ingots having a cross-sectional area of at least 1 m² and up to 4-8 m²can be manufactured according to this invention. Similarly, an upperlimit of the height of the ingot may be related to longer cycle times,and not the fundamentals of the casting process. Ingot heights of up toabout 50 cm to about 80 cm are possible. Thus, consistent with theinvention, a body of continuous monocrystalline silicon, ornear-monocrystalline silicon, can be successfully grown to about 66cm×66 cm in cross section, with a rectangular solid piece of continuousmonocrystalline silicon being at least 33,750 cm³ in volume. Further,consistent with the present invention, a solid body of cast continuousmonocrystalline silicon, or near-monocrystalline silicon, can be formedpreferably having at least two dimensions each being as large as theinterior dimensions of a casting vessel and the third dimension beingthe same height as the ingot. For example, if the cast body ofmonocrystalline silicon is a cube-shaped or a rectangular-shaped solid,these dimensions above would refer to the length, width, and height ofsuch bodies.

Similarly, a solid body of geometric multi-crystalline silicon,preferably cast geometric multi-crystalline silicon, and preferablyhaving at least two dimensions each being at least about 10 cm, and athird dimension at least about 5 cm, can be formed. Preferably, a solidbody of geometric multi-crystalline silicon, preferably cast geometricmulti-crystalline silicon, and having at least two dimensions each beingat least about 20 cm, and a third dimension at least about 5 cm, can beformed. More preferably, a solid body of geometric multi-crystallinesilicon, preferably cast geometric multi-crystalline silicon, and havingat least two dimensions each being at least about 30 cm, and a thirddimension at least about 5 cm, can be formed. Still more preferably, asolid body of geometric multi-crystalline silicon, preferably castgeometric multi-crystalline silicon, and having at least two dimensionseach being at least about 35 cm, and a third dimension at least about 5cm, can be formed. Still more preferably, a solid body of geometricmulti-crystalline silicon, preferably cast geometric multi-crystallinesilicon, and having at least two dimensions each being at least about 40cm, and a third dimension at least about 5 cm, can be formed. Still morepreferably, a solid body of geometric multi-crystalline silicon,preferably cast geometric multi-crystalline silicon, and having at leasttwo dimensions each being at least about 50 cm, and a third dimension atleast about 5 cm, can be formed. Still more preferably, a solid body ofgeometric multi-crystalline silicon, preferably cast geometricmulti-crystalline silicon, and having at least two dimensions each beingat least about 60 cm, and a third dimension at least about 5 cm, can beformed. Still more preferably, a solid body of geometricmulti-crystalline silicon, preferably cast geometric multi-crystallinesilicon, and having at least two dimensions each being at least about 70cm, and a third dimension at least about 5 cm, can be formed. Thus,consistent with the invention, a body of continuous geometricmulti-crystalline silicon can be successfully grown to about 66 cm×66 cmin cross section, with a rectangular solid piece of continuous geometricmulti-crystalline silicon being at least 33,750 cm³ in volume. Further,consistent with the present invention, a solid body of geometricmulti-crystalline silicon preferably cast geometric multi-crystallinesilicon, can be formed preferably having at least two dimensions eachbeing as large as the interior dimensions of a casting vessel. Forexample, if the cast body of geometric multi-crystalline silicon is acube-shaped or a rectangular-shaped solid, these dimensions above wouldrefer to the length, width, and height of such bodies.

By conducting the crystallization of the molten silicon in a mannerconsistent with embodiments of the invention, cast silicon havingspecific, rather than random, grain boundaries and specific grain sizescan be made. Additionally, by aligning the seed(s) in a manner such thatall seeds are oriented the same relative direction to each other, forexample the (100) pole direction being perpendicular to a bottom of thecrucible and the (110) pole direction parallel to one of the sides of arectangular or square cross-section crucible, large bodies of castsilicon can be obtained that are, or are nearly, monocrystalline siliconin which the pole direction of such cast silicon is the same as that ofthe seed(s). Similarly, other pole directions may be perpendicular tothe bottom of the crucible. Moreover, consistent with an embodiment ofthe invention, the seed(s) may be arranged so that any common poledirection is perpendicular to a bottom of the crucible.

When monocrystalline silicon is made by the conventional method ofpulling a cylindrically shaped boule from a pool of molten silicon,e.g., according to the CZ or FZ methods, the monocrystalline siliconobtained contains radially distributed impurities and defects, such asswirl defects (formed from intrinsic defects such as vacancies andself-interstitial atoms) and OSF ring defects. Swirl defects areinterstitial silicon atoms or vacancies, either in singular or clusteredform. Such swirl defects can be detected by x-ray topography and appearas “swirls” in the silicon. They can also be detected after preferentialacid etching of the silicon for defect delineation.

According to the conventional CZ or FZ methods, the distribution ofoxygen atoms within the silicon and the defects in the silicon caused bysuch oxygen atoms are radially situated. This means that they tend to bearranged in rings, spirals or striations that are symmetric about acentral axis. OSF ring defects are a particular example of this, wherenanometer-scale oxygen precipitates nucleate stacking faults in acylindrical band within a pulled monocrystalline ingot or boule ofsilicon, resulting in circular defect bands on wafers made from suchsilicon. Such bands can be observed in a sample of silicon afterpreferential acid etching.

Both swirl defects and OSF ring defects occur in the boules ofmonocrystalline silicon by pulling a cylindrically shaped boule from apool of molten silicon, e.g., according to the conventional CZ or FZmethods, due to the rotational symmetry of the pulling process, theaxial thermal gradients, and the rotation inherent in the process. Incontrast, silicon can be made by casting processes according toembodiments of the invention that does not exhibit such swirl defectsand OSF ring defects. This is because the incorporation of defectsduring the casting process can be essentially distributed randomly at agrowth interface not influenced by rotation, in a body of silicon thatdoes not possess cylindrical symmetry, and in a process where theisotherms are essentially flat across the ingot throughout thesolidification and the cool-down processes.

Concerning the concentrations of light element impurities in silicongrown by different methods, the following levels, shown in TABLE 1, arewidely considered characteristic.

TABLE 1 Concentrations (atoms/cm³) Oxygen Carbon Nitrogen Float Zone  <1× 10¹⁶ <1 × 10¹⁶ <1 × 10¹⁴ Czochralski 2 × 10¹⁷-1 × 10¹⁸ <1 × 10¹⁶ <5 ×10¹⁴ Cast 2-3 × 10¹⁷ 2 × 10¹⁶-5 × 10¹⁷ >1 × 10¹⁵

Parts of CZ ingots can be produced with as low as 5×10¹⁷ atoms/cm³ ofoxygen, but not lower. Carbon and nitrogen concentrations can beincreased in FZ and CZ ingots by intentional doping, but doping does notexceed the solid solubility limit in these techniques (as it does incast material), and doped ingots have not been made in sizes larger than20 cm diameter. By contrast, cast ingots are typically supersaturatedwith carbon and nitrogen due to release coatings and the design of thefurnace hot zone. As a consequence, precipitated nitrides and carbidesare ubiquitous due to liquid phase nucleation and growth. Furthermore,cast single crystal ingots have been manufactured, according toembodiments of the invention, with the above-reported impurity levelsand with sizes as large as 50×50×20 cm³ and 60×60×5 cm³. Thesedimensions are exemplary only, and are not considered upper limits forthe casting processes of the invention.

For example, regarding impurity levels, a dissolved carbon concentrationof about 1-5×10¹⁷ atoms/cm³ (notation for about 1×10¹⁷ atoms/cm³ toabout 5×10¹⁷ atoms/cm³), a dissolved oxygen concentration of about2-3×10¹⁷ atoms/cm³, and a dissolved nitrogen concentration of about1-5×10¹⁵ atoms/cm³ are preferred in the silicon cast according to thisinvention. According to embodiments of the present invention, a solidbody of geometric multi-crystalline silicon, preferably cast geometricmulti-crystalline silicon, and preferably having at least two dimensionseach being at least about 10 cm, and a third dimension at least about 5cm, can be formed, having a dissolved carbon concentration of about1-5×10¹⁷ atoms/cm³, a dissolved oxygen concentration of about 2-3×10¹⁷atoms/cm³, and a dissolved nitrogen concentration of about 1-5×10¹⁵atoms/cm³. Preferably, a solid body of geometric multi-crystallinesilicon, preferably cast geometric multi-crystalline silicon, and havingat least two dimensions each being at least about 20 cm, and a thirddimension at least about 5 cm, can be formed, having a dissolved carbonconcentration of about 1-5×10¹⁷ atoms/cm³, a dissolved oxygenconcentration of about 2-3×10¹⁷ atoms/cm³, and a dissolved nitrogenconcentration of about 1-5×10¹⁵ atoms/cm³. More preferably, a solid bodyof geometric multi-crystalline silicon, preferably cast geometricmulti-crystalline silicon, and having at least two dimensions each beingat least about 30 cm, and a third dimension at least about 5 cm, can beformed, having a dissolved carbon concentration of about 1-5×10¹⁷atoms/cm³, a dissolved oxygen concentration of about 2-3×10¹⁷ atoms/cm³,and a dissolved nitrogen concentration of about 1-5×10¹⁵ atoms/cm³.Still more preferably, a solid body of geometric multi-crystallinesilicon, preferably cast geometric multi-crystalline silicon, and havingat least two dimensions each being at least about 35 cm, and a thirddimension at least about 5 cm, can be formed, having a dissolved carbonconcentration of about 1-5×10¹⁷ atoms/cm³, a dissolved oxygenconcentration of about 2-3×10¹⁷ atoms/cm³, and a dissolved nitrogenconcentration of about 1-5×10¹⁵ atoms/cm³. Still more preferably, asolid body of geometric multi-crystalline silicon, preferably castgeometric multi-crystalline silicon, and having at least two dimensionseach being at least about 40 cm, and a third dimension at least about 5cm, can be formed, having a dissolved carbon concentration of about1-5×10¹⁷ atoms/cm³, a dissolved oxygen concentration of about 2-3×10¹⁷atoms/cm³, and a dissolved nitrogen concentration of about 1-5×10¹⁵atoms/cm³. Still more preferably, a solid body of geometricmulti-crystalline silicon, preferably cast geometric multi-crystallinesilicon, and having at least two dimensions each being at least about 50cm, and a third dimension at least about 5 cm, can be formed, having adissolved carbon concentration of about 1-5×10¹⁷ atoms/cm³, a dissolvedoxygen concentration of about 2-3×10¹⁷ atoms/cm³, and a dissolvednitrogen concentration of about 1-5×10¹⁵ atoms/cm³. Still morepreferably, a solid body of geometric multi-crystalline silicon,preferably cast geometric multi-crystalline silicon, and having at leasttwo dimensions each being at least about 60 cm, and a third dimension atleast about 5 cm, can be formed, having a dissolved carbon concentrationof about 1-5×10¹⁷ atoms/cm³, a dissolved oxygen concentration of about2-3×10¹⁷ atoms/cm³, and a dissolved nitrogen concentration of about1-5×10¹⁵ atoms/cm³. Still more preferably, a solid body of geometricmulti-crystalline silicon, preferably cast geometric multi-crystallinesilicon, and having at least two dimensions each being at least about 70cm, and a third dimension at least about 5 cm, can be formed, having adissolved carbon concentration of about 1-5×10¹⁷ atoms/cm³, a dissolvedoxygen concentration of about 2-3×10¹⁷ atoms/cm³, and a dissolvednitrogen concentration of about 1-5×10¹⁵ atoms/cm³.

An upper limit of horizontal size of an ingot of cast silicon madeaccording to embodiments of the invention, and having theabove-referenced impurity concentrations, is only determined by castingand crucible making technology, and not by the invented method itself.Thus, consistent with the invention, a body of continuous geometricmulti-crystalline silicon can be successfully grown to about 66 cm×66 cmin cross section, with a rectangular solid piece of continuous geometricmulti-crystalline silicon being at least 33,750 cm³ in volume. Further,consistent with the present invention, a solid body of geometricmulti-crystalline silicon preferably cast geometric multi-crystallinesilicon, can be formed preferably having at least two dimensions eachbeing as large as the interior dimensions of a casting vessel. Forexample, if the cast body of geometric multi-crystalline silicon is acube-shaped or a rectangular-shaped solid, these dimensions above wouldrefer to the length, width, and height of such bodies.

The seed(s) used for casting processes, consistent with embodiments ofthe invention, can be of any desired size and shape, but are suitablygeometrically shaped pieces of monocrystalline silicon,near-monocrystalline silicon, or geometrically ordered multi-crystallinesilicon, such as square, rectangular, hexagonal, rhomboid or octagonalshaped pieces of silicon. They can be shaped conducive to tiling, sothey can be placed or “tiled” edge-to-edge and conformed to the bottomof a crucible in a desired pattern. Also consistent with embodiments ofthe invention, seeds can be placed on one or more, including all, sidesof the crucible. Such seeds can be obtained, for example, by sawing asource of crystalline silicon, such as a boule of monocrystallinesilicon, into pieces having the desired shapes. The seeds can also beformed by cutting them from a sample of either continuousmonocrystalline, near-monocrystalline silicon, or continuous geometricmulti-crystalline silicon made by a process according to embodiments ofthe invention, such that seed(s) for use in subsequent casting processescan be made from an initial casting process. Thus, for example, a slabof either continuous monocrystalline or near-monocrystalline silicon cutor otherwise obtained from an ingot of continuous monocrystalline ornear-monocrystalline silicon can function as a template for a subsequentcasting of continuous monocrystalline or near-monocrystalline silicon.Such a seed crystal can be the size and shape, or substantially the sizeand shape, of a side, such as the bottom, of a crucible or other vesselin which the seed is placed. For the purposes of monocrystallinecasting, it is preferable to have as few seeds as possible to cover thecrucible bottom in order to avoid the incorporation of defects. Thus,the seed or seeds can be the size and shape, or substantially the sizeand shape, of one or more sides, such as the bottom, of a crucible orother vessel in which the seed or seeds is placed to perform the castingmethod in accordance with this invention.

Processes and apparatuses for preparing silicon in accordance withcertain embodiments of the invention will now be described. However, itis to be understood that these are not the only ways to form siliconconsistent with the principles of the invention.

Referring to FIG. 1, seeds 100 are placed at the bottom of a bottomedand walled crucible 110, such as a quartz crucible, in a way such thateither they closely abut in the same orientation so as to form a large,continuously oriented slab 120. Alternatively, they closely abut inpre-selected misorientations so as to produce specific grain boundarieswith deliberately chosen grain sizes in the resulting silicon that isproduced. That is, for casting of geometric multi-crystalline silicon,the cross-sectional grain size and, preferably, cross-sectional shape ofthe resulting crystallized geometric multi-crystalline silicon will beequal to or will approximate that of the seeds and the height of thegrain can be a long as the dimension of the silicon that isperpendicular to the cross-section. If a geometric multi-crystallineseed crystal, for example, a slab of geometric multi-crystalline siliconcut or otherwise obtained from an ingot of geometric multi-crystallinesilicon, is used a seed crystal or seed crystals for casting geometricmulti-crystalline silicon, the cross-sectional grain size and,preferably, cross-sectional shape of the grains of the resultinggeometric multi-crystalline silicon will approximate the grains in thegeometric multi-crystalline seed or seeds. Thus, a slab of geometricmulti-crystalline silicon cut or otherwise obtained from an ingot ofgeometric multi-crystalline silicon can be a “geometricmulti-crystalline silicon seed crystal” (also referred to as a“geometrically ordered multi-crystalline silicon seed crystal”), and canfunction as a template for a subsequent casting of geometricmulti-crystalline silicon. Such a seed crystal can be the size andshape, or substantially the size and shape, of a side, such as thebottom, of a crucible or other vessel in which the seed is placed. Whensuch a seed crystal is used in the method of this invention, theresulting geometric multi-crystalline silicon will preferably havecrystal grains that have the same or substantially the samecross-sectional size and shape as the grains in the seed. Preferably,seeds 100 are tiled and placed so as to substantially cover the entiretyof the bottom of crucible 110. It is also preferable that crucible 110has a release coating such as one made from silica, silicon nitride, ora liquid encapsulant, to aid in the removal of crystallized silicon fromcrucible 110. Further, the seeds may comprise a slab or slabs ofmonocrystalline silicon of a desired crystal orientation, about 3 mm toabout 100 mm thick. While a specific number and size of seeds 100 isshown in FIG. 1, it will be readily apparent to one of ordinary skill inthe art that both the number and size of the seeds can be increased ordecreased, depending on the application.

Referring to FIG. 2, seeds 100 can also be placed on one or more sidewalls 130, 140 of crucible 110. Seeds 100 can be placed on all fourwalls of crucible 110, although for illustration purposes only, seeds100 are shown only on walls 130, 140. Preferably, the seeds 100 that areplaced on any of the four walls of crucible 110 are columnar tofacilitate crystal growth. Preferably, each of the columnar seeds placedon any of the four walls of crucible 110 will have the same grainorientation as the seed placed immediately below it on the bottomsurface of crucible 110. In the case of geometric multi-crystallinesilicon growth, placing the columnar seeds in this manner willfacilitate the growth of geometric multi-crystalline silicon grains aslarge as the height of the crucible 110.

Still referring to FIG. 2, advantages of this arrangement of seeds 110are a quicker, more simple, self-propagating process for casting siliconwith higher crystallinity and higher growth rates. For example, siliconmay be melted in a silicon ‘cup’, consisting of many seeds that arestacked together to form a cavity, e.g., a bottom and four walls, insidecrucible 110. Alternatively, molten silicon may be poured in a silicon‘cup’, consisting of many seeds that are stacked together to form acavity, e.g., a bottom and four walls, inside crucible 110. In analternative example, the receiving ‘cup’ is first brought up to themelting temperature of silicon, but maintained in solid state, and thenthe molten silicon is poured in and allowed to come to thermalequilibrium. Then, in either example above, crucible 110 is cooled,whereby heat is removed from the bottom and sides of crucible 110 by,for example, a solid heat sink material (not shown) which radiates heatto the ambient, while heat is still applied to the open top of crucible110. In this way, the resulting cast ingot of silicon may be eithermonocrystalline or geometric multi-crystalline (depending on the type ofseeds 100 used and their orientation), and the crystallization proceedsfaster than known multi-crystalline casting processes. To repeat thisprocess, a portion of the sides and bottom of the crystallized siliconingot are removed, using known techniques, and can be reused in asubsequent casting process. Preferably, a plurality of seed crystals,e.g., seeds 100, are arranged so that a common pole direction amongseeds 100 is perpendicular to each of the bottom and a side of crucible110, so that no grain boundaries are formed between the bottom and aside of crucible 110.

FIGS. 3A-3C illustrate an example of tiling for casting geometricmulti-crystalline silicon in crucible 110. Crystal grain engineering canbe achieved by careful seed creation, orientation, placement, andcrystal growth. FIGS. 3A and 3B, for example, show two monocrystallinesilicon slabs 155, 165, on which different (110) directions areindicated. Both slabs have a common (100) direction perpendicular totheir surfaces. Each slab of monocrystalline silicon 155, 165, is thencut to form many pieces of silicon, which become seeds 150, 160. Thesurface types can be uniform, e.g., (100), for texturing reasons, orchosen at will. The shape and size of grains may be selected based onthe cutting of the tiles from slabs of monocrystalline silicon 155 and165, as shown in FIG. 3B. The relative orientation angles betweenneighboring tiles of pieces 150, 160, determines the grain boundary type(e.g., high angle, low angle or twin) in the resulting cast geometricmulti-crystalline silicon. In FIG. 3A, for example, two grainorientations of the (100) pole direction are shown.

The seeds shown in FIG. 3C are then comprised of tiled monocrystallinesilicon pieces 150, 160 that have specifically selected orientationrelationships with their neighboring tiles. Silicon pieces 150, 160 arethen tiled in the bottom of crucible 110, shown in FIG. 3C, such thatthe two (110) directions are alternating, as shown by the arrows drawnon pieces 150, 160. It is important to note that pieces 150, 160 aredrawn as roughly square blocks for illustrative purposes only, and forthe reasons discussed below, could be other shapes.

Although not shown in FIG. 3C, seeds may also be located on the sides ofcrucible, as in FIG. 2. Silicon feedstock (not shown) may then beintroduced into crucible 110 over pieces 150, 160, and then melted.Alternatively, molten silicon may be poured in crucible 110. In thealternative example, crucible 110 is first brought very close to or upto the melting temperature of silicon, and then the molten silicon ispoured in. Consistent with embodiments of the invention, a thin layer ofthe seeds can be melted before solidification begins.

Then, in either example above, crucible 110 is cooled, whereby heat isremoved from the bottom of crucible 110 (and sides only if seeds aretiled on the side surfaces as well) by, for example, a solid heat sinkmaterial which radiates heat to the ambient, while heat is still appliedto the open top of crucible 110. Thus, melted silicon is introducedwhile the seed is maintained as a solid, and directional solidificationof the melt causes the upwards growth of the columnar grains. In thisway, the resulting cast ingot of geometric multi-crystalline siliconwill mimic the grain orientations of tiled silicon seeds 150, 160. Oncethis technique is properly implemented, the resulting ingot can be cutinto, for example, horizontal slabs to act as seed layers for othercasting processes. The slab can have, for example, the size and shape,or substantially the size and shape, of a surface, such as a bottom, ofa crucible or other vessel used for the casting. For example, only onesuch slab can be used for a casting process.

FIG. 4 illustrates a variation of the tiling shown in FIG. 3C. As anexample of grain orientation for cast geometric multi-crystallinesilicon, seed pieces 150, 160 are tiled with a common pole direction(001) being perpendicular to the bottom of crucible 110. In FIG. 4, allvariations of the (110) family of directions are represented in thetiling of pieces 150, 160, as indicated by the directional arrows.Although not shown in this particular figure, seeds can also be on oneor more sides of crucible 110.

Thus, the orientation of seed crystals in a crucible used to form thesilicon may be chosen such that specific grain boundaries are formed incast geometric multi-crystalline silicon, and where such grainboundaries enclose geometric shapes. In contrast to embodiments of theinvention, known casting processes involve the casting ofmulti-crystalline grains in an uncontrolled fashion by directionalsolidification from a completely melted mass of silicon. The resultinggrains have basically random orientation and size distribution. Therandom grain orientation makes it difficult to effectively texture thesilicon surface. Furthermore, it has been shown that kinks in the grainboundaries, natural products of the typical growth techniques, tend tonucleate structural defects involving clusters or lines of dislocations.These dislocations, and the impurities that they tend to attract, causefast recombination of electrical carriers and the degradation ofperformance as a photovoltaic material. Therefore, consistent with anembodiment of the invention, careful planning and seeding of a regulargrain boundary network for casting of either monocrystalline orgeometric multi-crystalline silicon is accomplished such that the size,shape and orientation of grains is explicitly chosen to maximizeminority carrier lifetime and impurity gettering while minimizingstructural defects.

Grain boundaries can be chosen to be flat planes in order to minimizedislocation nucleation while maintaining their vertical direction duringgrowth. The grain boundary types are chosen to maximize gettering ofimpurities and stress relief. The grain orientations (and especially thesurface orientation) are chosen to allow texturing, improve surfacepassivation and enhance grain strength. The size of the grains is chosento optimize the balance between effective gettering distances and largeabsorption areas. For example, casting of geometric multi-crystallinesilicon can be accomplished such that the geometric multi-crystallinesilicon has an average minimum grain cross-section size of at leastabout 0.5 cm to about 10 cm with a common pole direction beingperpendicular to the surface of the cast geometric multi-crystallinesilicon, as shown, for example, in FIGS. 3C and 4. The average crystalgrain cross-section size can be about 0.5 cm to about 70 cm, or larger.As described above, the cross-section size of a grain of geometricmulti-crystalline silicon is understood as the longest dimension of thecross-section of the grain that is perpendicular to the height or lengthof the grain. The net result is an overall increase in efficiency of theresulting photovoltaic material.

Consistent with an embodiment of the invention, a geometric arrangementof a plurality of monocrystalline silicon seed crystals can be placed onat least one surface in a crucible, e.g., a bottom surface of acrucible, wherein the geometric arrangement includes close-packedpolygons. Alternatively, a geometric arrangement of a plurality ofmonocrystalline silicon seed crystals can be placed such that thegeometric arrangement includes close-packed hexagons, or polygonalshapes having rhomboid or triangular interstices, as shown, for example,in FIGS. 5 and 6. In yet another alternative, instead of using aplurality of monocrystalline seed crystals, a section or slab of siliconcut or otherwise obtained from an ingot produced in a prior casting ofgeometrically ordered multi-crystalline silicon can be used as a singleseed crystal for casting geometrically ordered multi-crystalline siliconin accordance with this invention. Such a single geometricmulti-crystalline silicon seed crystal can be the same size and shape,or substantially the same size and shape, as a surface of the crucibleor other vessel used to conduct the casting. More specifically, FIG. 5illustrates an example of a close-packed array of hexagons 170. Incontrast, FIG. 6 illustrates an example of an array of polygonal shapeshaving rhomboid or triangular interstices 180, 190. Both arrays arediscussed in more detail below. Any of the arrangements discussed aboveare also applicable to an embodiment for casting either a solid body ofmonocrystalline silicon, a solid body of near-monocrystalline silicon,or a solid body of geometric multi-crystalline silicon, where the seedcrystals are so placed on both the bottom and side surfaces of acrucible.

The silicon crystal grains produced by casting a body of geometricmulti-crystalline silicon, consistent with embodiments of the invention,may be grown in a columnar manner. Further, such crystal grains may havea cross section that is, or is close to, the shape of the seed fromwhich it is formed. When making silicon that has such specificallyselected grain boundaries, preferably the grain boundary junctions onlyhave three grain boundaries meeting at a corner. As shown in FIG. 5,hexagonal arrangements of seed crystals 170 are desirable for the tilingof seeds where the crystal orientation is such that the atoms in thehorizontal plane have three-fold or six-fold symmetry, such as (111) forsilicon. Thus, FIG. 5 illustrates a plan view of a portion of acollection of hexagonal-shaped seeds for arrangement in the bottom of asuitable crucible, such as that shown in FIGS. 1 and 2. The arrowsindicate the orientation of the (110) direction of the silicon crystalin the seeds.

Alternatively, for orientations with 4-fold symmetry, a differentgeometric configuration of the seeds can be used to maintain stable,symmetric grain boundaries across multiple grains while still meetingthe three grain boundary corner rule. For example, if θ is thedisorientation between the (110) direction and the primary sides of anoctagon with a (100) pole, and α is the apex angle of an interstitialrhombus, as shown in FIG. 6, all crystal grains will have a symmetricgrain boundary with respect to the (110) direction if α=90°−θ. In thisexample, all crystal grains have a (100) pole direction perpendicular tothe plane of the paper on which FIG. 6 is depicted. Thus, FIG. 6 is aplan view of a portion of a collection of octagonal-shaped seeds alongwith rhombus-shaped seeds 180, 190 for arrangement in the bottom of asuitable crucible, such as that shown in FIGS. 1 and 2. The arrowsindicate the orientation of the (110) direction of the silicon crystalin the seeds.

FIG. 7 is a flowchart depicting an exemplary method of making silicon,consistent with the present invention. Consistent with FIG. 7, method700 may begin by selecting monocrystalline silicon seed crystals formonocrystalline or geometric multi-crystalline silicon growth, andarranging the monocrystalline silicon seed crystals in a crucible (step705). Alternatively, a single slab cut or otherwise obtained from aningot of monocrystalline silicon or geometrically orderedmulti-crystalline silicon can be used as a single seed crystal. Next,silicon feedstock may be added to the crucible (step 710). The crucibleis then heated from the top while the bottom of the crucible is cooledfrom the bottom (either passively or actively, see step 715). Duringmelting, the melt stage of the silicon is monitored to track and controlthe position of the solid-liquid interface (step 720). The melt stage ofthe silicon is allowed to proceed until a portion of the monocrystallinesilicon seed crystals are melted (step 725). Once a desired portion ofthe monocrystalline silicon seed crystals are melted, the melt stage isended and the crystal growth stage begins (step 730). The crystal growthis allowed to continue unidirectionally and vertically within thecrucible until the silicon crystallization is complete (step 735). Ifthe seeds are arranged for geometric multi-crystalline silicon growth,the crystallization of step 735 will produce a geometricmulti-crystalline silicon ingot with columnar grains (step 740).Alternatively, if the seeds are arranged for monocrystalline silicongrowth, the crystallization of step 735 will produce a monocrystallinesilicon ingot (step 745). Finally, the ingot produced in either step 740or 745 is removed for further processing (step 750).

As shown in FIG. 8A, silicon feedstock 200 may be introduced to crucible210 containing seeds 220 in, for example, one of two ways. In the first,crucible 210 is loaded to full capacity with solid silicon feedstock200, suitably in the form of conveniently sized chunks, and the loadedcrucible 210 is placed in a casting station (not shown).

As shown in FIG. 8B, the thermal profile in crucible 210 is set up sothat the top of the silicon charge in crucible 110 is heated to melting,while the bottom is actively or passively cooled to maintain the solidphase of seeds 220 at the bottom of crucible 210, i.e., so that they donot float when feedstock 200 is melted. A solid heat sink material 230is in contact with a bottom of crucible 210 for radiating heat towater-cooled walls. For example, heat sink material 230 can be a solidblock of graphite, and can preferably have dimensions as large or largerthan the bottom of the crucible. Consistent with the invention, forexample, the heat sink material can be 66 cm by 66 cm by 20 cm, whenused with a crucible having a bottom surface that is 66 cm by 66 cm. Theside walls of crucible 210 are, preferably, not cooled in any way,provided that seeds 220 are located only on the bottom of crucible 210.If seeds 220 are located on the bottom and sides of crucible 210, thenheat sink material 230 would be placed on both the bottom and sides ofcrucible 210 for maintaining the desired thermal profile.

The melting phase of silicon feedstock 200 is closely monitored to trackthe position of the interface between the melted silicon and the seeds.Preferably, melt 240 (shown in FIG. 8B) proceeds until all of thefeedstock silicon 200 except for seeds 220 is completely melted, afterwhich seeds 220 are partially melted. For example, the heating can beclosely controlled such that the seeds 220 do not melt completely, bymaintaining a ΔT of about 0.1° C./min or less, as measured on an outsidesurface of the crucible, after reaching the melting temperature ofsilicon elsewhere in the crucible. Preferably, the heating can beclosely controlled by maintaining a ΔT of about 0.05° C./min or less, asmeasured on an outside surface of the crucible, after reaching themelting temperature of silicon elsewhere in the crucible. For example,consistent with the invention, the ΔT can be measured on an outsidesurface of the crucible between the crucible and a large block ofgraphite, and a dip-rod may be inserted into melt 240 to measure thedepth of the melt, in order to calculate the portion of seeds 220 thathave melted.

As shown in FIG. 8C, portion 250 illustrates a melted portion of thetotal thickness of seeds 220, below the melt 240. After a portion 250 ofseeds 220 are melted below melt 240, the melt stage is then quicklyended and the crystal growth stage is begun, wherein the heating at thetop of crucible 210 is decreased and/or the cooling of the bottom atheat sink material 230 is increased. As an example of this process, thechart shown in FIG. 8D illustrates melting of a portion 250 of seeds 220as a function of time. As shown in FIG. 8D, a portion of the seedshaving an initial thickness between 5 and 6 cm are gradually melteduntil just under 2 cm of solid seed remains. For example, the heatingcan be closely controlled such that the seeds 220 do not meltcompletely, by maintaining a ΔT of about 0.1° C./min or less, asmeasured on an outside surface of the crucible (e.g., through athermocouple mounted in the cooling block), after reaching the meltingtemperature of silicon elsewhere in the crucible. Preferably, theheating can be closely controlled by maintaining a ΔT of about 0.05°C./min or less, as measured on an outside surface of the crucible, afterreaching the melting temperature of silicon elsewhere in the crucible.At this point, the melt stage is then quickly ended and the crystalgrowth stage is begun, which is indicated by the comparative rise insolid thickness measured on the ordinate of the chart.

Then, as shown in FIG. 8E, seeded crystal growth continuesunidirectionally, and vertically, within crucible 210 until the siliconcrystallization is complete. The casting cycle finishes when thetop-to-bottom thermal gradient within crucible 210 is evened out. Then,the entire ingot 260 is slowly cooled down to room temperature. Forcasting of geometric multi-crystalline silicon, as shown in FIG. 8E,this seeded unidirectional growth produces columnar shaped grains 270having, generally, a horizontal cross section that is the shape of theindividual seed 220 over which it is formed. In this manner, the grainboundaries of the cast geometric multi-crystalline silicon can bepre-selected. Any of the previously discussed seeding patterns/tilingare applicable to this casting process.

Alternatively, for casting of monocrystalline silicon, the arrangementof seeds 220 can be made to have no grain boundaries at all, resultingin cast monocrystalline silicon. As shown in FIG. 8F, portion 250illustrates a melted portion of the total thickness of seeds 220, belowthe melt 240. After a portion 250 of seeds 220 are melted below melt240, the melt stage is then quickly ended and the crystal growth stageis begun, wherein the heating at the top of crucible 210 is decreasedand/or the cooling of the bottom at heat sink material 230 is increased.Then, as shown in FIG. 8G, seeded crystal growth continuesunidirectionally, and vertically, within crucible 210 until the siliconcrystallization is complete. A preferably substantially flatsolid-liquid interface 285 propagates upward and away from a bottomsurface of crucible 210. The casting cycle finishes after the completionof crystal growth, when the top-to-bottom thermal gradient withincrucible 210 is evened out. Then, the entire ingot 280 is slowly cooleddown to room temperature. For casting of monocrystalline silicon, asshown in FIG. 8G, this seeded unidirectional growth produces acontinuous solid body of cast monocrystalline silicon 290.

In another process, illustrated in FIG. 9, silicon feedstock 200 may befirst melted in a separate compartment or separate melt vessel 300.Seeds 220 may or may not be partially melted from the top before themolten feedstock 305 is fed or poured into crucible 210 via melt pipe310, after which cooling and growth proceeds as described with referenceto FIGS. 8B-8G. In another embodiment, silicon seed crystals may bemounted on the walls of crucible 210 (not shown) and seeded growth canproceed from the sides as well as the bottom of crucible 210, asdescribed previously. Alternatively, silicon feedstock 200 is melted ina melt vessel 300 separate from crucible 210, and at the same timecrucible 210 is heated to the melting temperature of silicon, and theheating is controlled so that seeds 220 do not melt completely. Uponpartial melting of seeds 220, molten feedstock 305 can be transferredfrom melt vessel 300 into crucible 210, and the cooling andcrystallization can begin. Thus, consistent with an embodiment of theinvention, a portion of the solid body of crystallized silicon caninclude seeds 220. Alternatively, the seeds may be kept completely solidprior to melt introduction. In this case, the molten silicon in meltvessel 300 is heated beyond the melting temperature, and the superheatedliquid is allowed to melt a portion of some of the seeds when thesuperheated liquid is introduced.

In a two-stage casting station, such as that shown in FIG. 9, moltenfeedstock 305 would pour down from melt vessel 300, land on seeds 220,and assume their crystallinity during solidification. Alternatively,melting may take place in a central melt vessel 300, which feeds adistributed arrangement of solidification crucibles, such as one or morecopies of crucible 210 (not shown). Consistent with embodiments of theinvention, the solidification crucibles can be lined with seeds 220 oneither or both of the sides and bottom of the crucibles. Some advantagesof this approach include: the separation of melting and solidificationsystems, to allow better optimization of each casting step; asemi-continuous melting of silicon, where melting of new material canoccur in a regular fashion, as needed to maintain the crucible supply;slagging of the top (and potential draining of the bottom) silicon whilethe solidification stations are fed from the middle of the melt,enhancing purity of the starting silicon material; and allowing meltvessel 300 to come into equilibrium with molten feedstock 305 and nolonger be a significant source of impurities.

Thus, after an ingot 260 or 280 has been cast by one of the methodsdescribed above, the resulting cast ingot can be processed further by,for example, cutting off the bottom or another section of the ingot andusing it as a single crystal seed in a subsequent casting run to form abody of monocrystalline silicon, near-monocrystalline silicon, orgeometric multi-crystalline silicon, consistent with the invention, andwherein the size and shape of such single crystal seed is the same sizeand shape of the bottom of crucible used in the subsequent casting run,and the rest of the ingot can be cut into bricks and wafers forprocessing into photovoltaic cells. Alternately, the entire ingot can becut into, for example, horizontal slabs for use as seed crystals inmultiple casting stations for future casting runs.

The silicon feedstock used in processes consistent with embodiments ofthe invention can contain one or more dopants such as those selectedfrom a list including: boron, aluminum, lithium, gallium, phosphorus,antimony, arsenic, and bismuth. The total amount of such dopant can beabout 0.01 parts per million by atomic % (ppma) to about 2 ppma.Preferably, the amount of dopant in the silicon is an amount such that awafer made from the silicon has a resistivity of about 0.1 to about 50ohm-cm, preferably of about 0.5 to about 5.0 ohm-cm.

Thus, consistent with the present invention, the silicon can be a bodyof cast continuous monocrystalline silicon, cast near-monocrystallinesilicon, or cast continuous geometric multi-crystalline silicon, thatpreferably is essentially free of, or free of, radially distributeddefects such as OSF's and/or swirl defects, and, preferably, where atleast two dimensions of the body are preferably at least about 10 cm,preferably at least about 20 cm, more preferably at least 30 cm, stillmore preferably at least 40 cm, still more preferably at least 50 cm,still more preferably at least 60 cm, and most preferably at least about70 cm. Most preferably, the third dimension of such a body of silicon isat least about 5 cm, preferably at least about 15 cm and most preferablyat least about 20 cm. The body of silicon can be one separate piece as asingle body, or it can be contained within or surrounded by, totally orpartially, other silicon. The body of silicon can be formed preferablyhaving at least two dimensions each being as large as the interiordimensions of a casting vessel. As disclosed herein, embodiments of theinvention can be used to produce large bodies of monocrystallinesilicon, near-monocrystalline silicon, or geometric multi-crystallinesilicon by a simple and cost-effective casting process.

The following are examples of experimental results consistent withembodiments of the invention. These examples are presented for merelyexemplifying and illustrating embodiments of the invention and shouldnot be construed as limiting the scope of the invention in any manner.

Example 1

Seed preparation: A boule of pure Czochralski (CZ) silicon(monocrystalline), obtained from MEMC, Inc. and having 0.3 ppma ofboron, was cut down along its length using a diamond coated band saw sothat it had a square cross section measuring from 14 cm per side. Theresulting block of monocrystalline silicon was cut through its crosssection using the same saw into slabs having a thickness of about 2 cmto about 3 cm. These slabs were used as monocrystalline silicon seedcrystals, or “seeds.” The (100) crystallographic pole orientation of thesilicon boule was maintained. The resulting single crystal silicon slabswere then arranged in the bottom of a quartz crucible so that the (100)direction of the slabs faced up, and the (110) direction was keptparallel to one side of the crucible. The quartz crucible had a squarecross section with 68 cm on a side, a depth of about 40 cm, and a wallthickness of about 1.8 cm. The slabs were arranged in the bottom of thecrucible with their long dimension parallel to the bottom of thecrucible and their sides touching to form a single, complete layer ofsuch slabs on the bottom of the crucible.

Casting: The crucible then was filled up to a total mass of 265 kg ofsolid silicon feedstock at room temperature. The filled crucible wasthen loaded into an in-situ melting/directional solidification castingstation used to cast multi-crystalline silicon. The melt process was runby heating resistive heaters to approximately 1550° C., and the heaterswere configured so that the heating came from the top while heat wasallowed to radiate out the bottom by opening the insulation a total of 6cm. This configuration caused the melting to proceed in a top-downdirection towards the bottom of the crucible. The passive coolingthrough the bottom caused the seed crystals to be maintained in solidstate at the melting temperature, as was monitored by a thermocouple.The extent of melting was measured by a quartz dip rod that was loweredinto the melt every ten minutes. The dip rod height was compared with ameasurement taken on an empty crucible in the station to determine theheight of the remaining solid material. By dip rod measurement, firstthe feedstock melted, and then the melting phase was allowed to continueuntil only a height of about 1.5 cm of the seed crystals remained. Atthis point, the heating power was dropped to a temperature setting of1500° C., while the radiation from the bottom was increased by openingthe insulation to 12 cm. One or two additional millimeters of seedcrystals melted before solidification began, as observed by dip-rodmeasurements. Then seeded single crystal growth proceeded until the endof the solidification step. The growth stage and the remainder of thecasting cycle was performed with the normal parameters where thetop-to-bottom thermal gradient was evened out, and then the entire ingotwas slowly cooled to room temperature. The cast silicon product was a 66cm by 66 cm by 24 cm ingot, of which a central portion having ahorizontal square cross section measuring 50 cm by 50 cm wasmonocrystalline silicon from top to bottom. The monocrystalline siliconstructure was evident from visually inspecting the surface of the ingot.Additionally, etching of the silicon with a caustic formula capable ofdelineating grain boundaries further affirmed the lack of grainboundaries in the material. The bulk doping average was 1.2 ohm-cm, andthe photovoltaic cells manufactured from this silicon had an electricalefficiency of 16.0%.

In other casting runs conducted in accordance with this example, it wasobserved that the cast silicon product was a contiguously consistentcrystal of silicon that contained smaller crystals of silicon of othercrystal orientations, or was a body of monocrystalline silicon that hadadjacent regions of multi-crystalline silicon.

Example 2

Seed preparation: Seeding was accomplished as in Example 1, except thatthe monocrystalline silicon seeds were cut so that the (110) directionwas at 45 degrees from the side of the square seeds for half of theseeds, while the other half had an angle of approximately 20 degrees.The square pieces were layered in the bottom of the crucible in acheckerboard manner alternating the two different seed orientations,i.e., the (110) direction had an angle of 45 degrees and 20 degrees fromthe orientation of the crucible sides. Relative to one another, theseeds had either 25 degrees or 155 degrees of misorientation. However,due to size mismatches of the square-shaped seeds, some gaps in theseeding layer were left uncovered. The crucible measured approximately33 cm on each of the square sides and approximately 22 cm tall.

Casting: The crucible containing the seeds and a separate cruciblecontaining a total of 56 kg of feedstock silicon chunks were loaded intoa Ubiquitous Casting Process (UCP) two-stage casting station. Thereceiving crucible (with the seeds inside) was heated up to the meltingpoint of silicon, but not given the energy to melt completely. Thesilicon in the other crucible was melted by resistive graphite heatersat a temperature at least 50° C. above the melting temperature ofsilicon, and then poured into the receiving crucible. At this point,solidification began immediately, with the heat being extracted from thebottom of the receiving crucible in order to effect directionalsolidification and seeded crystal growth. The standard growth cycle wasshortened to account for the mass of already solidified material thatthe seeds constituted. In this way, instead of allowing time for all 66kg (10 kg of seeds and 56 kg of feedstock silicon) to solidify beforethe cool down process began, only time for the 56 kg of molten siliconwas provided to avoid waste of heating energy. The product of thisprocess was an ingot of silicon with large, generally columnar grainshaving a square cross section having shape and dimensions that remainedclose to the top surface of the original seed crystal dimensions overwhich they were formed. The lateral grain boundary positions drifted insome cases as the growth proceeded.

Example 3

Seed preparation: Seeding was accomplished with 23 kg of square, (100),plates used to line the bottom of a crucible, providing a coverage areaof 63 cm×63 cm and a thickness ranging from 3 cm in the center to 1.8 cmat the sides. All plates were arranged with their (110) directions at45° from the walls of the crucible.

Casting: The crucible containing the seeds was filled with an additionaltotal of 242 kg of feedstock silicon chunks, representing a mix ofintrinsic silicon, silicon recycled from previous ingots, anddouble-cast silicon with a p-type resistivity greater than 9 ohm-cm. Thecharge of silicon in the crucible was loaded into a one-stagedirectional solidification furnace. The crucible (with the seeds inside)was heated up to a temperature of 1550° C., while the bottom was cooledby opening the insulation to 12 cm. The solid-liquid interface remainedsubstantially flat during melting, such that at the end of melting, nopart of the seed was melted through. The thickness of the silicon wasmonitored by use of a quartz dip rod. When a center thickness wasmeasured at 2.5 cm, the melt stage was stopped, the heater temperaturedropped to 1440° C. and the insulation height was increased to 15 cm.From the beginning of the melt phase change, the rate of temperatureincrease was maintained at or below 0.1° C./min, as measured on anoutside surface of the crucible, after reaching the melting temperatureof silicon elsewhere in the crucible. Then, the remainder of thesolidification process was allowed to proceed, with roughly constantpower to the heater being maintained until the end of crystal growth wasobserved. After the end of growth, the temperature of the crystallizedsilicon ingot was evened out and then brought uniformly down to roomtemperature. After removing the ingot from the crucible, the bottom ofthe ingot was cut off in one large piece for later re-use as a seed inanother subsequent casting process, and the remainder of the ingot wascut into 12.5 cm square bricks for further processing. The process wassuccessful in begetting monocrystalline growth substantially over theentire seed layer cross-section, and proceeded through to the top of theingot. Monocrystallinity was evident from inspection of the cut silicon.

In other casting runs conducted in accordance with this example, it wasobserved that the cast silicon product was a contiguously consistentcrystal of silicon that contained smaller crystals of silicon of othercrystal orientations, or was a body of monocrystalline silicon that hadadjacent regions of multi-crystalline silicon.

Wafers made from the silicon consistent with embodiments of theinvention are suitably thin and can be used in photovoltaic cells.Furthermore, the wafers can be n-type or p-type. For example, wafers canbe about 10 microns thick to about 700 microns thick. Further, thewafers used in the photovoltaic cells preferably have a diffusion length(L_(p)) that is greater than the wafer thickness (t). For example, theratio of L_(p) to t is suitably at least 0.5. It can, for example, be atleast about 1.1, or at least about 2. The diffusion length is theaverage distance that minority carriers (such as electrons in p-typematerial) can diffuse before recombining with the majority carriers(holes in p-type material). The L_(p) is related to the minority carrierlifetime τ through the relationship L_(p)=(Dτ)^(1/2), where D is thediffusion constant. The diffusion length can be measured by a number oftechniques, such as the Photon-Beam-Induced Current technique or theSurface Photovoltage technique. See for example, “Fundamentals of SolarCells”, by A. Fahrenbruch and R. Bube, Academic Press, 1983, pp. 90-102,for a description of how the diffusion length can be measured.

The wafers can have a width of about 100 millimeters to about 600millimeters. Preferably, the wafers have at least one dimension being atleast about 50 mm. The wafers made from the silicon of the invention,and consequently the photovoltaic cells made by the invention can, forexample, have a surface area of about 50 to about 3600 squarecentimeters. The front surface of the wafer is preferably textured. Forexample, the wafer can be suitably textured using chemical etching,plasma etching, or laser or mechanical scribing. If a wafer having a(100) pole orientation is used, the wafer can be etched to form ananisotropically textured surface by treating the wafer in an aqueoussolution of a base, such as sodium hydroxide, at an elevatedtemperature, for example about 70° C. to about 90° C., for about 10 toabout 120 minutes. The aqueous solution may contain an alcohol, such asisopropanol.

Thus, solar cells can be manufactured using the wafers produced fromcast silicon ingots according to the embodiments of the invention, byslicing the solid body of cast silicon to form at least one wafer;optionally performing a cleaning procedure on a surface of the wafer;optionally performing a texturing step on the surface; forming a p-njunction, for example, by doping the surface; optionally depositing ananti-reflective coating on the surface; optionally forming at least onelayer selected from a back surface field and a passivating layer by, forexample, an aluminum sintering step; and forming electrically conductivecontacts on the wafer. A passivating layer is a layer that has aninterface with a bare wafer surface that ties up the dangling bonds ofthe surface atoms. Examples of passivating layers on silicon includesilicon nitride, silicon dioxide and amorphous silicon. This layer isgenerally thinner than one micron, either being transparent to light oracting as an anti-reflective layer.

In a typical and general process for preparing a photovoltaic cellusing, for example, a p-type silicon wafer, the wafer is exposed on oneside to a suitable n-dopant to form an emitter layer and a p-n junctionon the front, or light-receiving side of the wafer. Typically, then-type layer or emitter layer is formed by first depositing the n-dopantonto the front surface of the p-type wafer using techniques commonlyemployed in the art such as chemical or physical deposition and, aftersuch deposition, the n-dopant, for example, phosphorus, is driven intothe front surface of the silicon wafer to further diffuse the n-dopantinto the wafer surface. This “drive-in” step is commonly accomplished byexposing the wafer to high temperatures. A p-n junction is therebyformed at the boundary region between the n-type layer and the p-typesilicon wafer substrate. The wafer surface, prior to the phosphorus orother doping to form the emitter layer, can be textured. In order tofurther improve light absorption, an optional anti-reflective coating,such as silicon nitride, can be typically applied to the front of thewafer, sometimes providing simultaneous surface and or bulk passivation.

In order to utilize the electrical potential generated by exposing thep-n junction to light energy, the photovoltaic cell is typicallyprovided with a conductive front electrical contact on the front face ofthe wafer and a conductive back electrical contact on the back face ofthe wafer, although both contacts can be on the back of the wafer. Suchcontacts are typically made of one or more highly electricallyconducting metals and are, therefore, typically opaque.

Thus, solar cells consistent with the embodiments described above maycomprise a wafer formed from a body of continuous monocrystallinesilicon or near-monocrystalline silicon being free or substantially freeof radially-distributed defects, the body can be as describedhereinabove, and, for example, having at least two dimensions each beingat least about 25 cm and a third dimension being at least about 20 cm, ap-n junction in the wafer, an optional anti-reflective coating on asurface of the wafer; preferably having at least one layer selected froma back surface field and a passivating layer; and electricallyconductive contacts on the wafer, wherein the body may be free orsubstantially free of swirl defects and free or substantially free ofOSF defects.

Also, solar cells consistent with the embodiments described above maycomprise a wafer formed from a body of continuous geometricmulti-crystalline silicon, the body having a predetermined arrangementof grain orientations, preferably with a common pole direction beingperpendicular to a surface of the body, the body preferably furtherhaving at least two dimensions each preferably being at least about 10cm, a p-n junction in the wafer; an optional anti-reflective coating ona surface of the wafer, preferably having at least one layer selectedfrom a back surface field and a passivating layer, and electricallyconductive contacts on the wafer, wherein the geometricmulti-crystalline silicon includes silicon grains having an averagecrystal grain cross-section size of about 0.5 cm to about 30 cm, andwherein the body may be free or substantially free of swirl defects andfree or substantially free of OSF defects.

It will be apparent to those skilled in the art that variousmodifications and variations can be made in the disclosed structures andmethods without departing from the scope or spirit of the invention. Forexample, the disclosed processes and methods that relate to formingmonocrystalline silicon are also applicable to formingnear-monocrystalline silicon, geometric multi-crystalline silicon, orcombinations thereof. Moreover, although casting of silicon has beendescribed herein, other semiconductor materials and nonmetalliccrystalline materials may be cast without departing from the scope andspirit of the invention. For example, the inventor has contemplatedcasting of other materials consistent with embodiments of the invention,such as germanium, gallium arsenide, silicon germanium, aluminum oxide(including its single crystal form of sapphire), gallium nitride, zincoxide, zinc sulfide, gallium indium arsenide, indium antimonide,germanium, yttrium barium oxides, lanthanide oxides, magnesium oxide,calcium oxide, and other semiconductors, oxides, and intermetallics witha liquid phase. In addition, a number of other III-V or II-VI materials,as well as metals and alloys, could be cast according to embodiments ofthe present invention. Other embodiments of the invention will beapparent to those skilled in the art from consideration of thespecification and practice of the invention disclosed herein. It isintended that the specification and examples be considered exemplaryonly, with a true scope and spirit of the invention being indicated bythe following claims.

1. A method of casting one or more of a semiconductor, an oxide, and anintermetallic material, comprising: placing a geometric arrangement of aplurality of monocrystalline seed crystals on at least one surface in acrucible having one or more side walls heated to at least the meltingtemperature of the one or more materials, and at least one wall forcooling; placing a molten form of the one or more materials in contactwith the geometric arrangement of monocrystalline seed crystals; andforming a solid body comprising a geometrically orderedmulti-crystalline form of the one or more materials, optionally havingat least two dimensions each being at least about 10 cm, by cooling themolten form of the one or more materials to control crystallization,wherein the forming includes controlling a solid-liquid interface at anedge of the molten form of the one or more materials during the coolingso as to move in a direction that increases a distance between themolten form of the one or more materials and the at least one wall forcooling.
 2. A method of casting one or more of a semiconductor, anoxide, and an intermetallic material, comprising: arranging a pluralityof monocrystalline seed crystals in a predetermined pattern on at leasttwo surfaces of a crucible having one or more side walls heated to atleast the melting temperature of the one or more materials and at leastone wall for cooling; placing a molten form of the one or more materialsin contact with the plurality of monocrystalline seed crystals; andforming a solid body comprising a geometrically orderedmulti-crystalline form of the one or more materials, optionally havingat least two dimensions each being at least about 10 cm, by cooling themolten form of the one or more materials from the at least two surfacesof the crucible to control crystallization, wherein the forming includescontrolling a solid-liquid interface at an edge of the molten form ofthe one or more materials during the cooling so as to move the interfacein a direction that increases a distance between the molten form of theone or more materials and the monocrystalline seed crystals in thecrucible.
 3. A method of casting one or more of a semiconductor, anoxide, and an intermetallic material, comprising: placing a geometricarrangement of a plurality of seed crystals on at least one surface in acrucible; placing feedstock of the one or more materials in contact withthe plurality of seed crystals on the at least one surface; heating thefeedstock and the plurality of seed crystals to the melting temperatureof the one or more materials; controlling the heating so that theplurality of seed crystals does not melt completely, the controllingcomprising maintaining a ΔT of about 0.1° C./min or less, as measured onan outside surface of the crucible, after reaching the meltingtemperature of the one or more materials elsewhere in the crucible; and,once the plurality of seed crystals are partially melted, forming asolid body comprising a geometrically ordered multi-crystalline form ofthe one or more materials by cooling the one or more materials.
 4. Amethod of casting one or more of a semiconductor, an oxide, and anintermetallic material, comprising: arranging a plurality of seedcrystals in a predetermined pattern on at least two surfaces of acrucible; placing feedstock of the one or more materials in contact withthe plurality of seed crystals on the at least two surfaces; heating thefeedstock and the plurality of seed crystals to the melting temperatureof the one or more materials; controlling the heating so that theplurality of seed crystals does not melt completely, the controllingcomprising maintaining a ΔT of about 0.1° C./min or less, as measured onan outside surface of the crucible, after reaching the meltingtemperature of the one or more materials elsewhere in the crucible; and,once the plurality of seed crystals are partially melted, forming asolid body comprising a geometrically ordered multi-crystalline form ofthe one or more materials by cooling the one or more materials.
 5. Amethod of casting one or more of a semiconductor, an oxide, and anintermetallic material, comprising: placing at least one geometricmulti-crystalline seed crystal on at least one surface in a cruciblehaving one or more side walls heated to at least the melting temperatureof the one or more materials and at least one wall for cooling; placinga molten form of the one or more materials in contact with the at leastone seed crystal; and forming a solid body comprising a geometricallyordered multi-crystalline form of the one or more materials, optionallyhaving at least two dimensions each being at least about 10 cm, bycooling the molten form of the one or more materials to controlcrystallization, wherein the forming includes controlling a solid-liquidinterface at an edge of the molten form of the one or more materialsduring the cooling so as to move in a direction that increases adistance between the molten form of the one or more materials and the atleast one geometric multi-crystalline seed crystal in the crucible.
 6. Amethod of casting one or more of a semiconductor, an oxide, and anintermetallic material, comprising: placing a geometric arrangement of aplurality of seed crystals on at least one surface in a crucible, theplurality of seed crystals arranged to cover an entire or substantiallyan entire area of the at least one surface in the crucible; placing amolten form of the one or more materials in contact with the geometricarrangement of seed crystals; and forming a solid body comprising ageometrically ordered multi-crystalline form of the one or morematerials, optionally having at least two dimensions each being at leastabout 10 cm, by cooling the molten form of the one or more materials tocontrol crystallization.
 7. A method of casting one or more of asemiconductor, an oxide, and an intermetallic material, comprising:placing a molten form of the one or more materials in contact with atleast one geometrically ordered multi-crystalline seed crystal in avessel having one or more side walls heated to at least the meltingtemperature of the one or more materials, the at least one geometricallyordered multi-crystalline seed crystal arranged to cover an entire orsubstantially an entire area of a surface of the vessel; and forming asolid body comprising a geometrically ordered multi-crystalline form ofthe one or more materials, optionally having at least two dimensionseach being at least about 10 cm, by cooling the molten form of the oneor more materials to control crystallization. 8-21. (canceled)
 22. Themethod according to claim 1, wherein one or more of a semiconductor, anoxide, and an intermetallic material is selected from the groupconsisting of geranium, gallium arsenide, silicon germanium, aluminumoxide, sapphire, gallium nitride, zinc oxide, zinc sulfide, galliumindium arsenide, indium antimonide, germanium, yttrium barium oxides,lanthanide oxides, magnesium oxide, and calcium oxide
 23. The methodaccording to claim 2, wherein one or more of a semiconductor, an oxide,and an intermetallic material is selected from the group consisting ofgermanium, gallium arsenide, silicon germanium, aluminum oxide,sapphire, gallium nitride, zinc oxide, zinc sulfide, gallium indiumarsenide, indium antimonide, germanium, yttrium barium oxides,lanthanide oxides, magnesium oxide, and calcium oxide.
 24. The methodaccording to claim 3, wherein one or more of a semiconductor, an oxide,and an intermetallic material is selected from the group consisting ofgermanium, gallium arsenide, silicon germanium, aluminum oxide,sapphire, gallium nitride, zinc oxide, zinc sulfide, gallium indiumarsenide, indium antimonide, germanium, yttrium barium oxides,lanthanide oxides, magnesium oxide, and calcium oxide.
 25. The methodaccording to claim 4 wherein one or more of a semiconductor, an oxide,and an intermetallic material is selected from the group consisting ofgermanium, gallium arsenide, silicon germanium, aluminum oxide,sapphire, gallium nitride, zinc oxide, zinc sulfide, gallium indiumarsenide, indium antimonide, germanium, yttrium barium oxides,lanthanide oxides, magnesium oxide, and calcium oxide.
 26. The methodaccording to claim 5, wherein one or more of a semiconductor, an oxide,and an intermetallic material is selected from the group consisting ofgermanium, gallium arsenide, silicon germanium, aluminum oxide,sapphire, gallium nitride, zinc oxide, zinc sulfide, gallium indiumarsenide, indium antimonide, germanium, yttrium barium oxides,lanthanide oxides, magnesium oxide, and calcium oxide.
 27. The methodaccording to claim 6, wherein one or more of a semiconductor, an oxide,and an intermetallic material is selected from the group consisting ofgermanium, gallium arsenide, silicon germanium, aluminum oxide,sapphire, gallium nitride, zinc oxide, zinc sulfide, gallium indiumarsenide, indium antimonide, germanium, yttrium barium oxides,lanthanide oxides, magnesium oxide, and calcium oxide.
 28. The methodaccording to claim 7, wherein one or more of a semiconductor, an oxide,and an intermetallic material is selected from the group consisting ofgermanium, gallium arsenide, silicon germanium, aluminum oxide,sapphire, gallium nitride, zinc oxide, zinc sulfide, gallium indiumarsenide, indium antimonide, germanium, yttrium barium oxides,lanthanide oxides, magnesium oxide, and calcium oxide.